1. Field of the Invention
The present invention generally relates to a spectrum analyzer and, more particularly, to a spectrum analyzer which can easily detect the frequency spectrum of a BURST (where BURST means a signal having an envelope amplitude-modulated by signal such as a pulse like signal) signal by means of continuous frequency-sweep of the BURST signal.
2. Description of the Related Art
Most conventional spectrum analyzers are designed to analyze an input signal which last longer than its sweep time. Therefore, they cannot be applied to the analysis of BURST signals which last only for a short period of time.
Published Unexamined Japanese Patent Application No. 62-189669 discloses a spectrum analyzer designed to analyze burst signals. This spectrum analyzer performs frequency sweep of a burst signal input to it, measures the frequency spectrum of the burst signal, stores the data representing the frequency spectrum into a memory, reads the data, when necessary, and displays the frequency spectrum of the burst signal. This spectrum analyzer, known as "digital-storage spectrum analyzer (DSSA)," performs frequency sweep for every period the burst signal contains a high-frequency component and does not perform the sweep for every period the signal contains no high-frequency components. The period during which the burst signal has a high-frequency component, and the period during which the signal has no high-frequency components are far shorter than the period during which the spectrum analyzer analyzes long-lasting signals. The conditions in the analyzer remain the same, regardless of whether or not the frequency sweep is carried out.
The local oscillator incorporated in the DSSA is driven for every period the burst signal contains a high-frequency component, and not driven for every period the signal contains no high-frequency components. The detector, also incorporated in this analyzer, detects the burst signal for every period the signal contains a high-frequency component, and holds the detected data for every period the signal contains no high-frequency components.
The conventional DSSA is disadvantageous in the following respects:
(i) The performance stability of the local oscillator, and the sweep voltage and the like which control the local oscillator affect the accuracy of measuring the frequency (or frequency axis) of the signal. The accuracy of measuring the frequency greatly decreases with time. Due to the recurring periods during which the burst signal contains no high-frequency components, the accuracy of measuring is considerably low. Further, the higher the sweep speed, the more excessively the local oscillator will respond to the changes in the sweep voltage. (The sweep speed is either the ratio of the change in the sweep voltage to the sweep time, or the ratio of change in the sweep frequency to the sweep time, and the sweep speed changes every time the frequency sweep is started or stopped.) PA1 (ii) It is necessary to generate a sweep voltage having a waveform appropriate for performing the frequency sweep during every period the burst signal contains a high-frequency component and for not performing the frequency sweep during every period the burst signal contains no high-frequency components. PA1 (iii) Of the conditions for measuring the frequency of the burst signal, at least the resolution band width, the range of sweep frequency, and the sweep time must be displayed to the operator, because these conditions influence the accuracy of measuring. However, a correct sweep time cannot be displayed since, during what is referred to as "sweep time" in the prior art, there are in fact periods during which the sweep is not actually performed. The so-called sweep time includes the periods during which the frequency sweep is not actually carried out. The actual sweep time cannot be calculated and displayed, without a very complex circuit. PA1 high-frequency signal processing means for frequency-converting a signal to be measured including measurement object waves recurring for a short period of time, into an intermediate-frequency signal, by using a local oscillation signal swept continuously over a desired range of frequencies, and thereby outputting a frequency spectrum of the signal to be measured; PA1 detector means for detecting the frequency spectrum output by said high-frequency signal processing means; PA1 selection signal supplying means for supplying a selection signal representing a desired portion of the spectrum in which lies the measurement object waves; PA1 control means for outputting said portion of the spectrum demodulated by the demodulation means, in accordance with the selection signal supplied by the selection signal supplying means; PA1 display means for displaying the frequency spectrum output by the control means; and PA1 sweep control means for causing the high-frequency signal processing means to sweep the local oscillation signal repeatedly, each time for a period which is not an integral multiple of the cycle period of the selection signal. PA1 Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.